The present invention involves preparation of hydrophilic cross-linked polymeric materials which can be used for the fabrication of mass transfer agents such as membranes which allow selective transfer of water to or from a stream. The membranes are, for example, useful for the dehydration of organic solvent streams containing water by means of pervaporation or vapor permeation. The hydrophilic cross-linked polymeric materials described herein are comprised of two or more hydrophilic organic polymers containing at least 10% of at least one polyalkyl amine and at least one polyalcohol. These polymers are joined together using a cross-linking agent to form a cross-linked polymeric network.
Many situations require the transfer of water to or from an industrial process stream. Examples of such applications include: the drying of a gas prior to introducing that gas in a reactor to avoid undesired side reactions; the removal of water produced in a chemical reaction to drive the reaction to completion; the humidification of a gas stream to avoid drying moisture sensitive materials; and the dehydration of organic solvent streams to meet product specifications. The controlled transfer of water is carried out using hydrophilic mass transfer agents in the form of membranes or sorbent particles.
Alcohols, in particular 2-propanol (isopropyl alcohol—IPA), are being increasingly utilized in various industries as solvents and cleaning agents. Purification of alcohol streams when contaminated by water at certain concentrations becomes difficult due to the formation of azeotropic mixtures wherein the concentration of aqueous and organic components in the vapor and liquid phases are in equilibrium. Such mixtures cannot be separated by normal distillation, but only through complicated processes. Frequently, an additional substance is added to break the azeotropic mixtures. This additional substance must subsequently be completely removed and recovered from both product streams. An easy, efficient recovery and reuse of alcohols is needed to meet economic requirements and environmental regulations.
Pervaporation and vapor permeation are membrane-based unit operations in which water-free organic solvents can be produced as final product in a simple, and energy-effective manner. In pervaporation, water from a contaminated organic stream is preferentially transported across a thin membrane film. The source side of the membrane is wetted with the organic solvent/water liquid mixture, while vacuum or a sweep gas is used on the sink side of the membrane. The water is collected from the sink side by condensation. Vapor permeation is similar to pervaporation with one major difference—a vapor instead of a liquid contacts the source side of the membrane. In contrast to other membrane filtration processes, pervaporation/vapor permeation works according to a solution-diffusion mechanism. In microfiltration or ultrafiltration, for example, porosity is the key to preferential transport, and the flux rate depends upon molecular size. In pervaporation/vapor permeation, molecular interaction between membrane and separated species is the determining factor rather than the molecular size. The main component of the pervaporation/vapor permeation process is the membrane material which determines the permeation and selectivity and hence the separation properties of the process.
For pervaporation/vapor permeation to be economical and efficient, ultra-thin non-porous hydrophilic films of appropriate polymers need to be deposited onto a porous support matrix. Such a combination will provide high throughput along with good mechanical stability and will thus result in achieving the desired separation using minimum membrane area. Since water needs to be transported across the membrane, a high trans-membrane flow hydrophilic membrane must be used. The trans-membrane flow is a function of the composition of the feed. It is usually given as permeate amount per membrane area and per unit time, i.e. kg/m2-h, for the better permeating component. A further essential criterion for the suitability of the pervaporation membrane is its chemical and thermal stability. To obtain a high trans-membrane flow and a sufficient driving force, it is necessary to operate the pervaporation process at the highest possible temperatures. This however means that the membrane will be in contact with a feed mixture at high temperature, which has a high concentration of organic components, for example, organic solvents. To achieve an economical lifetime of the membranes, all components of the membrane must be long durable under these aggressive conditions.
Polyvinyl alcohol (PVA) membranes are widely used in dehydration pervaporation processes. The PVA membrane shows good selectivity towards water and is considered to have excellent film-forming characteristics, with a good resistance to many organic solvents, but it has poor physical stability in aqueous mixtures. Generally, two methods of treatment are employed to improve the stability of PVA in aqueous solution: cross-linking and crystallization. The post-cross-linking procedure often involves heat treatment of the PVA solid film for a certain time. As PVA is a semi-crystalline polymer, crystallization will occur during the heat treatment as well. Crystalline regions hinder the migration of solvent molecules through the membrane due to its impermeability and its physical cross-linking effect. As a result, permeability decreases rapidly with the increase in the degree of crystallinity in a PVA membrane. Although the selectivity is more or less increased after heat treatments, the loss in permeability is generally much higher and can override the gain in selectivity in the overall performance. On the other hand, polyalkyl amines, such as polyallyl amine hydrochloride (PAA), are typically more hydrophilic than PVA. However, extremely hydrophilic polymeric materials tend to swell significantly when water is present. Such swelling results in higher fluxes through the membrane, but also results in a drastic reduction in selectivity. In this invention, cross-linked polymers of polyalkyl amines, such as PAA, with polyalcohols, such as PVA, exhibit both high fluxes and high selectivities.
U.S. Pat. No. 6,093,686, “Liquid for contact lenses”, Nakada and Matano, Jul. 25, 2000 describes an aqueous solution of PAA used as preservative solution for contact lenses. U.S. Pat. No. 6,224,893, “Semi-interpenetrating or interpenetrating polymer networks for drug delivery and tissue engineering”, Langer et al., May 1, 2001 describes compositions for tissue engineering and drug delivery based on a mixture of polymerizable materials including PVA and PAA. The two polymers are not cross-linked together; instead they form interpenetrating or semi-interpenetrating polymer networks. U.S. Pat. No. 6,441,089, “Water-Soluble Polymers and Compositions Thereof”, Smith et al., Aug. 27, 2002 teaches chelating polymers that are water soluble, i.e. not cross-linked to create an interconnected matrix. U.S. Pat. No. 6,525,113 B1, “Process for Producing Cross-linked-Polyallylamine Hydrochloride”, Klix et al., Feb. 25, 2003 teaches PAA that is was not blended with PVA to create a cross-linked polymeric material. None of the compositions and methods of the prior art used as a semi-permeable membranes for those organic processes described above. Furthermore, whereas PAA and PVA are mentioned as possible polymers in the prior art, such art does not claim specific cross-linked polymer matrices of such polymers.
U.S. Pat. No. 6,099,621, “Membranes Comprising Aminoacid Salts in Polyamine Polymers and Blends,” W. S. Winston Ho, Aug. 8, 2000 teaches use of a polyamine (such as PAA) or polyamine blended with another polymer (such as PAA blended with PVA) with at least one aminoacid salt present. U.S. Pat. No. 6,099,621 requires the presence of at least one salt of an aminoacid in the range of 10 to 80 wt %, whereas the present invention does not require an aminoacid salt. In fact, the aminoacid salt might be detrimental to the objective for which the present invention is useful. The composition of U.S. Pat. No. 6,099,621 is not used as a semi-permeable dehydrating material or as water sorbing gel.
Patent application Ser. No. 10/145,838 of Vane, et al. teaches use of PVA with minimal amounts of PAA cross-linked in a composition containing silicon dioxide. However, at the low levels of polyalkyl amines disclosed therein, the membranes are quite inferior to those described herein.